melabs U2 programmer won't program 12F675

[BrianT]

I can program fresh SMD PIC12F675 with the melabs U2 USB programmer via the melabs 8/14 pin programming adapter but that assumes loose chips before soldering to the board. I have my candle board set up for ICSP and I have a short, ~35 mm, cable between the U2 and the board with the 12F675 on it. That cable is the exact one I used when I first built the boards and I do not recall ever having any problems. Now it definitely does not work so I suspect there may have been some timing tweaks in the melabs programmer code that render my ICSP lead invalid.

I will remove all the PIC12F675 from the boards and reprogram them off board.

Thanks for your help.

Cheers
Brian

[ardhuru]

Brian, if fresh devices program ok, and you have a problem only with re-programming, try enabling Vpp-before-Vdd in your programming software. This is a common problem with these devices if you are using the internal oscillator, AND have disabled mclr.

[BrianT]

Hi Ardhuru,
Thanks for the tip but I am not sure I can use it. On a scope I can see Vpp jumps from zero to about 14 volts about 100 microseconds before switched Vdd comes up. Is that what you mean? I can't change timing inside the melabs U2 programmer.

I have tried powering the target PCB three ways.
One was to power the PIC via the melabs programmer Vdd line. They give a caution that current is limited but I am only powering the PIC chip and there are no other components on the board. The programming voltage reads 13.8 with Vdd 4.88.
Two was to power the PIC via switched Vdd. Again no other components on the board.
Three was to power the target board with the normal 5 volt supply and then attempt programming.

I am sure the target PIC is getting erased as I can do a READ and see all memory cells are 3fff.

When I hit 'program' the device appears to be taking code but on verify it gives me a 'code programming error at location 0000'.

I give up.

BrianT